Create CONTRIBUTING.md

CONTRIBUTING.md

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+The project is run on Github, in the [typical](http://producingoss.com) free software way - we'd love to accept your patches and contributions to this project. There is just one thing you need to do first.
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+Contributions to Google projects, even unofficial ones like this, must be accompanied by a Contributor License Agreement. This is not a copyright assignment, it simply gives Google permission to use and redistribute your contributions as part of the project.
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+If you are an individual writing original source code and you're sure you own the rights, then you'll need to sign an
+[individual CLA](https://developers.google.com/open-source/cla/individual).
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+If you work for a company that wants to allow you to contribute your work, then you'll need to sign a [corporate CLA](https://developers.google.com/open-source/cla/corporate).
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+You generally only need to submit a CLA once, so if you've already submitted one for a different project, you probably don't need to do it again.
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+After your contribution is included, you will be listed in CONTRIBUTORS.txt and/or AUTHORS.txt: CONTRIBUTORS is the official list of people who can contribute (and typically have contributed) code to this repository, while the AUTHORS file lists the copyright holders.